The classical double copy, first presented in [1]}, maps classical Yang-Mills solutions to classical gravity solutions via a `squaring' procedure. This procedure is based on the color-kinematics duality for amplitudes (see [2]}, [3]}, [4]}). As in color-kinematics duality, the classical gravitational solution is built from two copies of the classical Yang-Mills solution, so the Yang-Mills solution is called the `single copy'. Since the original formulation eight years ago, many other examples of the classical double copy have been built [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]}. Recent reviews include [29]} and chapter 7 of [3]}.
